If you have studied Raoult’s Law you will be familiar
with the graph of vapour pressure vs composition at constant temperature. You will probably also be familiar with the boiling
point/composition diagram (at constant pressure) that is used to explain fractional distillation. What is often absent from
the books is the link between the two diagrams.

Here it is.

As temperature increases, so does the vapour pressure of a liquid.

The vapour pressure/ composition diagram is plotted at constant temperature

a series of these plots at different temperatures(fig 1 ) is needed in order to obtain a boiling point/composition diagram.

The graph shown is for a mixture of hexane and heptane, which forms an ideal liquid
mixture.

Since vapour pressure does not increase linearly with an increase in temperature, the
vapour pressure lines at the different temperatures are not parallel.

Boiling-point composition diagrams are usually plotted at atmospheric pressure, though
for an ideal liquid mixture they’d look more or less the same whatever the chosen pressure.

The horizontal line is at 1 atm (760 mmHg), and gives the composition of the liquid
that gives rise to a vapour pressure of 1 atm at each temperature.

Boiling-point composition diagrams are usually plotted
at atmospheric pressure

For an ideal liquid mixture they’d look more
or less the same whatever the chosen pressure.

The horizontal line is at 1 atm (760 mmHg), and gives
the composition of the liquid that gives rise to a vapour pressure of 1 atm at each temperature.

The points generated are then transferred to
axes having the same composition scale but whose vertical axis is the temperature (fig 2).

The boiling point/composition diagram is now completed
by putting in the line that represents the vapour composition (shown in red).

The vapour in equilibrium with liquid at a given temperature
is always richer in the more volatile component.

The vapour composition is shown by a point at the same
temperature,

Its composition is nearer to the more volatile
(lower boiling temperature) constituent.

The link between the vapour pressure/composition and
boiling temperature/composition diagrams for non-ideal (azeotropic) mixtures is the same,

Such systems give a more complex diagram because of
the vapour pressure maximum or minimum.